The Highway Safety Manual (HSM) provides guidance on the application of crash prediction models for network screening, evaluation of alternative designs, and evaluation of implemented safety improvements. A variety of models are available. They may be classified as network screening level (or simple models), project level [or crash modification factor (CMF)–corrected models], and evaluation-level models. The use of crash prediction models to identify sites with promise of safety improvement is often based on assessing the difference between the expected number of crashes ( Ne) at a site and the predicted number of crashes ( Np) for similar sites within the population. A large difference between Ne and Np may denote a safety problem and be used to identify and rank sites with promise. The HSM indicates that CMF-corrected models may be used for network screening purposes. However, issues arise in the analysis regarding the definition of similar sites. When CMF-corrected models are used, the definition of similar sites changes with changes in the CMF adjustments particular to each site. Use of ( Ne − Np) for identifying and ranking sites with promise does not work well when the analysis uses a CMF-corrected estimate of Np. A large combined CMF will increase Np and thereby decrease the value ( Ne − Np). But a large combined CMF may point to a site with promise. This paper examines this phenomenon in detail, reviews a case study, and suggests that simple models may be preferable for identifying sites with promise.
